1) Field of the Invention
The present invention relates generally to video displays and their associated driving circuits and in particular to LCD video display column driving circuits that use a multiplexing arrangement to reduce the number of input video data lines and that also use data lines and pixel capacitors that are precharged prior to the application of the video data signals to enable selected ones to be discharged to an appropriate level by the incoming video data signals to enhance the operation of the display.
2) Description of Related Art
Matrix display devices commonly utilize a plurality of display elements that are arranged in a matrix of rows and columns and supported on opposing sides of a thin layer of electro-optic material. Switching devices are associated with the display elements to control the application of data signals thereto. The display elements include a pixel capacitor driven by a transistor as a switching device. One of the pixel electrodes is on one side of the matrix display and a common electrode for each of the pixels is formed on the opposite of the matrix display. The transistor is usually a thin-film transistor (TFT) that is deposited on a transparent substrate such as glass. The switching element transistor has its source electrode connected to the pixel electrode that is deposited on the glass on the same side of the display matrix as the switching transistor. The drain electrodes of all of the switching transistors in a given column are connected to the same column conductor to which data signals are applied. The gate electrodes of all of the switching transistors in a given row are connected to a common row conductor to which row selection signals are applied to switch all the transistors in a selected row to the ON condition or state. By scanning the row conductors with the row selection signals, all of the switching transistors in a given row are turned ON and all of the rows are selected in a sequential fashion. At the same time, video data signals are applied to the column conductors in synchronism with the selection of each row. When the switching transistors in a given row are selected by the row select signal, the video data signals supplied to the switching transistor electrodes cause the pixel capacitors to be charged to a value corresponding to the data signal on the column conductor. Thus each pixel with its electrodes on opposite sides of the display acts as a capacitor. When the signal for a selected row is removed, the charge in the pixel capacitor is stored until the next repetition when that row is again selected with a row select signal and new voltages are stored therein. Thus a picture is formed on the matrix display by the charges stored in the pixel capacitors.
It is to be understood that the use of the term "video" herein, although it has been generally applied to the use of signals for television, is intended to cover displays other than TV pictures or displays. Such displays may be hand-held games having an LCD display with moving figures thereon and the like.
The resolution of the picture that is developed depends upon the number of pixels forming the image. It is common in a commercially available black and white active matrix liquid crystal display that is unscanned to have a display with 1024 columns and 768 rows. Such display requires 1792 row and column driver leads.
It is clear that the greater the number of pixels in a matrix, the more difficult it is to couple the many required column and row drive lines to the display. Thus a number of devices have been developed in an effort to reduce the number of connections required between the circuits external to the matrix and the circuitry deposited on the matrix itself. U.S. Pat. No. 4,922,240 discloses a proposal to integrate the scanner electronics on the display substrate using the same technology used in the manufacture of the pixel drivers for the LCD elements. It further proposed to reduce the number of connections to the matrix by using a commutator or switch configuration based on the same matrix configuration used in the active display to select an individual pixel. Operation for use as a TV display is not described.
U.S. Pat. No. 5,151,689 discloses a display device having a reduced number of column signal lines by using a switching arrangement that connects at least two display elements to a signal line in each row and sequentially scanning each row so that the display signal is time serially applied through the same signal line to each of at least two display elements connected to that signal line. Thus the total number of signal lines can be reduced to a value equal to or smaller than the number of display elements in the row direction.
U.S. Pat. No. 4,931,787 proposes to reduce the number of address conductors by arranging the picture elements in groups of at least two picture elements with the picture elements of each group being addressed with the same switching signal and data conductors. The switching transistors associated with the pixel elements of each group are operable at respective different voltage levels of the switching signal. Therefore, by using switching signals obtained from the driving means whose voltage levels change in predetermined manner over a selected amplitude range, the switching transistors associated with the picture elements of each group can be selectively controlled. In this way, one conductor can have several different voltages applied thereto which will operate a like number of pixels.
Other than these known examples, almost all of the commercially available active matrix liquid crystal displays are unscanned. Such unscanned display requires one external lead for each column and row line. As stated earlier, a direct line interface driver for a black and white 768.times.1024 computer display would require 1792 leads. Dealing with this many leads in the display drivers is an enormous problem as indicated earlier. It is a problem that will get worse as the resolution and complexity of the displays increase. Two major goals for solving the problem are to reduce the number of required input leads and to integrate the driver circuitry consisting of shift registers, latches and drivers directly onto the display substrate. This would reduce costs and increase reliability by eliminating the need for mounting integrated circuits on a separate substrate.